An automatic transaction apparatus has held therein a banknote classifying unit that includes a thickness detecting sensor that detects the thickness of a banknote. The banknote is classified by determining feature values of the banknote. The thickness value of the banknote is one of the feature values. The thickness value of a banknote is detected to determine the presence of multi-feeding or adhesion of a foreign body.
FIG. 7A illustrates an example of a conventional paper sheet thickness detecting sensor. Inserting a paper sheet KM between a fixed roller 32 and a detection roller 31, which is in intimate contact with the fixed roller 32 owing to a pressure from a shaft, shifts the detection roller 31 by an amount that corresponds to the thickness of the paper sheet KM. This shift amount is detected by shift sensors 33 and amplified to a predetermined level by an amplifier 34. The physical thickness can be determined, and the mechanism accuracy is expected to provide highly reliable information as feature information. The entirety of the surface of a paper sheet can be detected using a roller that is as wide as the paper sheet; however, when only a small number of shift sensors 33 are used, a mechanism is needed for slightly shifting the entirety of the detection roller 31, and it is difficult to provide such a mechanism. Providing multiple detection rollers 31 enables detection of the thickness of the entirety of a paper sheet, but this is unsuitable for providing a multichannel configuration due to the limitations of the size of the mechanism. Moreover, due to the mechanical structure, a jam fault, such as clogging by wastepaper, can easily occur.
FIG. 7B illustrates another example of a conventional paper sheet thickness detecting sensor. A transportation roller 42 transports a paper sheet KM in the direction indicated by an arrow to a place facing, across a transportation path, a plurality of detection electrodes 44a, 44b, 44c, and 44d and a signal electrode 43 to which a high-frequency signal with a predetermined frequency from an oscillator 41 is applied. To precisely detect the thickness of the paper sheet KM in a transportation path width direction of the transportation path, a plurality of amplifiers 45a, 45b, . . . , 45d are connected to the plurality of detection electrodes 44a, 44b, 44c, and 44d. Individual signals amplified by the plurality of amplifiers 45a, 45b, . . . , 45d are input to a multiplexer 46, and information on the output levels of the amplifiers 45a, 45b, . . . , 45d is converted from parallel data to serial data. An impedance between the electrodes is detected that changes in accordance with whether a paper sheet KM is on the transportation path and with the thickness of the paper sheet KM. The impedance is detected as another physical value that is different from thickness, and hence the detected information should not be easily trusted; however, the entirety of the surface of the paper sheet can be detected by disposing many detection electrodes 44 without providing a mechanism for the detection. This allows a multichannel configuration to be readily achieved and makes a jam fault unlikely to occur, in comparison with a method in which a plurality of detection rollers 31 are used. The conventional paper sheet thickness detecting sensor depicted in FIG. 7B detects the thickness of a paper sheet KM that corresponds to the position of detection electrodes in the width direction of the transportation path using the level of a high-frequency signal with a predetermined frequency, and thus needs as many amplifiers as the number of detection electrodes.
Patent document 1 below proposes a method of detecting the thickness of a paper sheet by detecting a change in an AC magnetic field generated by a plurality of detection coils provided above a plurality of detection rollers arranged with no gaps therebetween. Patent document 2 below proposes a method wherein a plurality of detection electrode systems are arranged in a direction crossing the direction of scanning of a paper medium, the detection electrode of each system is connected to an independent resonator, and an oscillatory frequency signal from a common oscillation circuit is applied to the resonators, so as to measure the thickness of the paper medium.
Patent document 1: Japanese Laid-open Patent Publication No. 2012-160060
Patent document 2: Japanese Laid-open Patent Publication No. 05-052504
However, in the conventional method depicted in FIG. 7B, i.e., a method of detecting the thickness of a paper sheet KM using a high-frequency signal, increasing the number of detection electrodes that sandwich the paper sheet KM with the signal electrode for the purpose of improving the resolution ability for thickness detection of the paper sheet KM leads to the need to provide an amplifier 45 for each of the plurality of detection electrodes and to install wiring for the amplifiers 45. Hence, increasing the resolution ability in the conventional method of detecting the thickness of a paper sheet KM using a high-frequency signal results in increased material costs and an increase in the fabrication cost due to the wiring.